I think you're looking for this plug-in from kiwidude which generally does the trick:
https://www.mobileread.com/forums/sho...d.php?t=134249
It's the method I use most frequently for this, anyway.
You can also do a tweak epub and substitute your image's file name for the name of the cover file in the html of the epub...also, you can open it with sigil and make the swap there, similarly. The plug-in method is the non-technical method though.